Follow-up bundle from owasp-docker removal / maturity: removed refactor.

scripts/tests/collections/Validate-Collections.Tests.ps1:786 already covers the basic case (It 'Detects maturity conflict with canonical collection'). Extend coverage to include:

• Conflicts across N>2 themed collections (e.g., three collections each declaring a different maturity for the same item).
• Combinations involving
  emoved vs stable,
  emoved vs deprecated, and deprecated vs �xperimental.
• Confirmation that the canonical hve-core-all collection is excluded from conflict detection (regression guard).
• Clear error-message assertions for each scenario.

Goal: prevent silent maturity drift as more themed collections adopt the maturity: removed marker.